Method for producing a solid body including a microstructure

ABSTRACT

According to a method for producing a solid body ( 1 ) including a microstructure ( 2 ), the surface of a substrate ( 3 ) is provided with a masking layer ( 6 ) that is impermeable to a substance to be applied. The substance is then incorporated into the substrate regions not covered by the masking layer ( 6 ). A heat treatment is used to diffuse the substance into a substrate region covered by the masking layer ( 6 ) such that a concentration gradient of the substance is created in the substrate region covered by the masking layer ( 6 ), proceeding from the edge of the masking layer ( 6 ) inward with increasing distance from the edge. The masking layer ( 6 ) is then removed to expose the substrate region under this layer, and a near-surface layer of the substrate ( 3 ) in the exposed substrate region is converted by a chemical conversion reaction into a coating ( 9 ) which has a layer thickness profile corresponding to the concentration gradient of the substance contained in this near-surface layer. A supplementary treatment is implemented in a subsection of the coating ( 9 ) in which the thickness of the coating ( 9 ) is reduced.

[0001] The invention relates to a method for producing a solid body including a microstructure, especially, a semiconductor element, wherein the surface of a substrate is provided with a masking layer which is impermeable to a substance to be applied, and wherein the substance is subsequently incorporated into substrate regions not covered by the masking layer.

[0002] A process of this type is known from the book Integrierte Digitalbausteine [Integrated Digital Components], Siemens AG (1970), pages 12 and 13. According to this process, to produce a semiconductor element, a surface region of a silicon substrate is covered by a masking layer which consists of silicon dioxide and is impermeable to a dopant, while other surface regions remain exposed. To create the masking layer, the substrate is first placed in a stream of oxygen where a continuous silicon dioxide layer is formed on the surface of the substrate. A light-sensitive photoresist is then applied to the substrate surface. This photoresist is exposed through a photomask which transmits light at those sites at which the substrate is to remain exposed for doping. After exposure, the photoresist is removed with a solvent from the exposed sites, while the unexposed regions of the photoresist which are insoluble by the solvent remain on the substrate. An etching agent is then used to etch away the silicon dioxide from the resist free sites, after which the remaining photoresist is removed. The substrate is then exposed at a temperature of approximately 1000° C. to a gas phase containing the dopant, during which the dopant diffuses into the open substrate sites not covered by the silicon dioxide. When the substrate cools, the diffusion process ceases. The substrate has then been doped regionally at the intended sites. The method may be used, for example, to integrate transistors, diodes, or similar electronic functional elements into the substrate.

[0003] The previously known method has the disadvantage, however, that the costs of the exposure device required to expose the substrate increase significantly as the size of the microstructures to be produced decrease—see F&M, Volume 107 (1994), Number 4, pages 57-60, and Number 9, pages 40-44. A principal disadvantageous aspect is that the resolution of the exposure device must be dimensioned for the smallest structure to be produced on the substrate even when large structures are simultaneously generated on the substrate. The production of solid bodies with the smallest structures is therefore complex and expensive.

[0004] The problem to be solved is therefore to create a method of the type referenced at the outset which provides cost-effective production of a solid body with a small structure.

[0005] This problem is solved by a process wherein the applied substance is diffused into a substrate layer covered by the masking layer in such a way that a concentration gradient of the substance is created, proceeding from the edge of the masking layer inward with increasing distance from the edge, in the substrate region covered by the masking layer; wherein the masking layer is next removed to expose the substrate region below it; wherein a layer of the substrate near the surface in the exposed substrate region is converted by a chemical conversion reaction into a coating with a layer thickness profile corresponding to the concentration gradient of the substance contained in this near-surface layer; and wherein a supplementary treatment is implemented in a subsection of the coating, the surface of which is smaller than the substrate surface covered by the original masking layer and in which the thickness of the coating is reduced relative to the remaining subsections of the coating, in which treatment the substrate region covered by this subsection is exposed, and/or a material is incorporated into this substrate region through the coating.

[0006] As a result of this heat treatment, the region of the substrate including the incorporated substance is thus enlarged, the substance being subdiffused underneath the edge of the masking layer. A concentration gradient with a locus-dependent concentration of the substance is created in the substrate region covered by the masking layer, with the concentration decreasing in the substrate plane running along the interface of masking layer and substrate, and proceeding from the edge of the masking layer inward with increasing distance from the edge. The coating which was created, after removal of the masking layer, by the chemical conversion reaction of the substrate region originally covered by the masking layer has a thickness at different sites of the substrate layer which corresponds to the concentration of the substance at the respective site. Depending on the chemical conversion reaction selected, the layer thickness of the coating along the substrate plane may either decrease or increase, proceeding from the edge of the substrate layer originally covered by the masking layer toward the interior of this substrate layer. Appropriate chemical conversion reactions are already well known. The supplementary treatment dependent on the layer thickness may be advantageously implemented for the substrate in a given region which is smaller that the region originally covered by the masking layer. During the supplementary treatment of the coating for example, the entire surface of the coating facing away from the substrate may be removed until a subsection of the substrate region originally covered by the coating is exposed at the sites in which the original thickness of the coating was smaller than at the remaining sites of the coating. In addition, however, the supplementary treatment may also incorporate a chemical substance into a subsection of the substrate region covered by the coating through the coating, for example by diffusion or bombardment with particles. In this process, the layer thickness profile of the coating is matched to the diffusion properties of the substance and/or to the kinetic energy of the particles in such a way that the substance is able to penetrate the coating only regionally at those sites where the layer thickness does not exceed a specified thickness.

[0007] In a masking layer which was created on the substrate by a photolithographic process, a structure may be produced, the dimensions of which are smaller than the dimensions of the smallest substrate surface still to be masked off from the light or still to be exposed, due to the limited resolution of the exposure device used for the photolithographic process. As a result, a cost-effective exposure device may be advantageously employed which has a lower resolution than that required for the dimensions of the smallest structure to be produced. The method is especially well suited for producing solid bodies that have both small and large structures.

[0008] In an advantageous embodiment of the invention, the substrate regions laterally adjoining the masking layer are covered with an etching mask and the masking layer is then contacted with an etching agent; the etching mask is preferably created by a chemical reaction in which a near-surface layer of the substrate regions to be covered by the etching mask is converted into an etching mask material. The etching mask may be applied to the surface regions not covered by the masking layer in a simple manner and without the use of a supplementary photolithographic step. To accomplish this, the near-surface layer may, for example, be converted in a nitrogen atmosphere to a nitride layer resistant to the specific etching agent. The entire surface of the solid body may then be contacted with the etching agent to remove the masking layer. In the event the etching mask is of a greater thickness than the masking layer, another etching agent may be employed which removes the etching mask in addition to the masking layer from the solid body. In this case, the etching rates and the thicknesses of the masking layer and etching mask must be matched to each other in such a way that, after the complete removal of the masking layer by the etching agent, the etching mask still possesses a residual thickness so as to continue to cover the substrate.

[0009] It is especially advantageous if the etching mask is created during the heat treatment in an oxygen-containing atmosphere while the substrate material is undergoing oxidation. This allows an additional fabrication step to be eliminated in the production of the etching mask.

[0010] It is advantageous if the chemical conversion reaction is an oxidation reaction. The coating may then be generated easily in an oxygen-containing atmosphere and, if required, with the input of energy. In the process, especially in the case of a silicon substrate into which a dopant has been diffused, a clear shaping of the layer profile of the coating is obtained as a function of the concentration gradient of the dopant in the substrate material.

[0011] In an advantageous embodiment of the invention, the near-surface layer of the substrate is converted by a chemical conversion reaction into an electrically insulating coating in the substrate region in which the masking layer has been removed; after the regional removal of the coating, a metal coating is electrolytically deposited on the exposed surface of the electrically conductive substrate region. As a result, it is possible, for example, to apply a microelectrode and/or a conductive track of small dimensions onto the substrate. The electrodeposition of the metal coating may be implemented specifically by a currentless technique.

[0012] It is advantageous for a preferably metallic surface layer to be applied to the surface of the solid body, and for the adhesive properties of the substrate material and of the coating to be matched to the material of the surface layer such that this layer continues to adhere only to the exposed subsection of the substrate region. The material of the surface layer here is selected so as to adhere more effectively to the exposed subsection of the surface region than to adjacent surface regions of the coating. Any layer regions adhering to the adjacent surface regions after coating may then, for example, be mechanically etched from the surface of the solid body, while the region of the surface layer adhering to the exposed subsection of the substrate region continues to adhere to this region. If required, the surface layer may also be mechanically stressed by the incorporation of impurities. When the layer regions adhering to the coating are detached, cracks may form along the periphery of the surface of the exposed subsection of the substrate region, the cracks facilitating the removal of the regions of this surface layer adhering to the coating.

[0013] In one embodiment of the invention, the near-surface layer of the substrate is converted by a chemical conversion reaction to a coating which is impermeable to the chemical substance to be applied; during the supplementary treatment, the substrate region covered by a subsection of this coating is first exposed, then the substance is incorporated into this substrate region. Incorporation of the substance, which may in particular be a dopant for the semiconductor substrate, may, for example, be implemented by diffusion or bombardment with particles—with the substance penetrating the exposed substrate region, while in those regions of the substrate covered by the coating, any penetration of the substance into the substrate is prevented by the coating.

[0014] In one embodiment of the invention, a heat treatment is used to diffuse the incorporated substance into a substrate region covered by the masking layer such that, proceeding from the edge of the masking layer inward with increasing distance from the edge of the masking layer, a concentration gradient of the substance is created in the substrate region covered by the masking layer; the masking layer is subsequently removed to expose the substrate region below it; a layer of the substrate near the surface in the exposed substrate region is converted by a chemical conversion reaction into a coating with an appropriate layer thickness profile corresponding to the concentration gradient of the substance contained in this near-surface layer; and a supplementary treatment is implemented in a subsection of the coating, the surface of which is smaller than the substrate surface covered by the original masking layer and in which the thickness of the coating is reduced relative to the remaining subsections of the coating, in which treatment the substrate region covered by this subsection is exposed, and/or a chemical substance is incorporated into this substrate region through the coating. The medium, the material of the coating laterally adjacent to the exposed substrate region, and/or the reaction conditions are preferably selected such that no chemical reaction occurs between the medium and the material of the coating. The chemical reaction is then limited to the exposed subsection of the substrate region such that this section may be chemically modified in a targeted fashion.

[0015] In one embodiment of the invention, after exposure of the substrate region, the substrate region is contacted with an etching agent for the substrate material, to which the coating surrounding the substrate region is essentially chemically resistant, in order to insert a depression in the substrate region. The coating then forms an etching mask for the etching agent. An anisotropic etching agent may be used to insert a groove with a roughly V-shaped cross-section into the substrate region. The solid body may be a component of a microreactor, the etched depression forming, for example, a supply channel for a substance to be inserted into the chamber of the microreactor, and/or forming a discharge channel for a substance to be discharged from the chamber. A metallic material is preferably used as the substrate for one component of the microreactor, for example aluminum or silver, which material provides effective dissipation of heat from or into the chamber of the microreactor. The following discussion explains the invention in more detail based on the drawings. The drawings show the following, part of which is in highly schematic form:

[0016]FIG. 1 is a cross-section through a solid body provided to produce a semiconductor element, into the substrate of which doping regions are incorporated laterally on both sides of the masking layer;

[0017]FIG. 2 shows the solid body seen in FIG. 1 after a heat treatment in which the doping material is diffused underneath the masking layer;

[0018]FIG. 3 shows the solid body seen in FIG. 2 after removal of the masking layer and subsequent application of a coating including a thickness profile;

[0019]FIG. 4 shows the solid body seen in FIG. 3 after the etching process in which the coating has been removed regionally from the substrate;

[0020]FIG. 5 shows the solid body seen in FIG. 4 after the selective application of a metal coating; and

[0021]FIG. 6 is a cross-section through a DMOS transistor cell.

[0022] In a method for producing a solid body 1 with a microstructure 2 in the form of a semiconductor element, a substrate 3 consisting preferably of silicon is provided, which substrate has passivation layers 4 consisting preferably of silicon dioxide on its surface, the layers covering substrate 3 continuously. Using a known method such as photolithographic application of an etch-resistant mask and followed by a wet etching process, an opening 5 is incorporated into the passivation layer 4, the opening exposing a subsection of substrate 3. To produce a masking layer 6, a silicon nitride layer is applied through opening 5 by a coating method such as chemical vapor deposition, over the entire substrate region exposed in opening 5. Subsequently, an etching mask resistant to an etching agent such as phosphoric acid is applied to this layer by a photolithographic procedure, which mask covers certain regions of the silicon nitride layer. Solid body 1 is then contacted with the etching agent to remove those regions of the silicon dioxide layer not covered by the etching mask. The etching mask is then removed. It is evident in FIG. 1 that the regions of the silicon nitride layer remaining on substrate 3 form masking layer 6 which covers a subsection of the substrate region located in opening 5, and that this masking layer 6 is spaced laterally on both sides of passivation layer 4. The material of masking layer 6 is selected based on its impermeability to a substance provided for doping the substrate, for example boron or phosphorus.

[0023] After masking layer 6 is produced, this substance is inserted into opening 5 to dope the substrate regions not covered by masking layer 6. This may be accomplished for example by exposing solid body 1 to a gas stream containing the substance. The substance then diffuses into the substrate regions not covered by masking layer 6 where it forms doping zones 7 (FIG. 1).

[0024] After and/or during the incorporation of the substance into doping regions 7, a heat treatment is implemented in which the incorporated substance is diffused into a substrate region covered by masking layer 6. The heat treatment may be performed at a temperature of, for example, 1000° C. It is clearly evident in FIG. 2 that doping regions 7 have expanded relative to FIG. 1 and that the dopant has sub-diffused underneath the edge of masking layer 6. Upon completion of the heat treatment, there is a decrease in the concentration of the substance in the coverage plane of doping regions 7, proceeding from masking layer 6 into the substrate region covered by masking layer 6 and with increasing distance from the edge of the masking layer.

[0025] During the heat treatment, solid body 1 is exposed to an oxygen-containing atmosphere in which an oxide layer is deposited in the opening onto the substrate region not covered by masking layer 6, the oxide layer forming an etching mask 8 which is resistant to an etching agent, such as phosphoric acid, used to remove masking layer 6. After completion of the heat treatment, masking layer 6 is contacted with this etching agent to remove masking layer 6, thus exposing the substrate region located under masking layer 6.

[0026] Next, a layer of substrate 3 located under the exposed substrate region is converted by a chemical conversion reaction in an oxygen-containing atmosphere into a silicon dioxide-coating 9. The local thickness of this coating 9 is dependent on the concentration of the substance diffused into the specific substrate region taking part in the chemical conversion reaction. It is clearly evident in FIG. 3, that the thickness of coating 9 decreases proceeding from the edge of coating 9 toward the center of coating 9, specifically, in accordance with the respective decrease in concentration of the substance in substrate 3.

[0027] In the embodiment of FIG. 4, coating 9 is exposed to an etching agent which etches away the material from the surface of coating 9 facing away from substrate 3. The etching process is stopped when a subsection of coating 9, in which the original thickness of coating 9 relative to the adjacent subsections of the original coating 9 is reduced, is completely removed, and substrate 3 covered by this subsection is exposed. It is evident from FIG. 4 that, after completion of the etching process, the substrate region covered by original masking layer 6 remains covered by coating 9 only along its peripheral regions, and that a substrate region which is smaller than the substrate region covered by original masking layer 6 has been exposed. While near-surface layers are also removed during the etching of coating 9 from passivation layer 4 and etching mask 91, the thickness of passivation layer 4 and that of etching mask 91 are adjusted to be large enough only part of their thickness to be etched away, and, as a result, the substrate material located beneath them continues to remain covered after completion of the etching process.

[0028] In the embodiment of FIG. 5, coating 9 consists of an electrically insulating material. After regional removal of coating 9 ¹, a metal layer 10 is electrolytically deposited on the exposed surface of substrate 3, which layer may form, for example, an electrode or a conductive track. It is clearly evident from FIG. 5 that the dimensions a of metal layer 10 are smaller than the dimensions b of original masking layer 6. The method may thus be employed to produce a microstructure 2, the dimensions of which are smaller than the resolution of an exposure device used to produce a photolithographically applied masking layer 6. As a result, the additional costs otherwise required for a high-resolution exposure device may be eliminated.

[0029] A substance may be incorporated into the substrate region exposed by the regional removal of coating 9. The material of coating 9 is selected based on the fact that the residual amount of coating 9 remaining on substrate 3 after exposure of the substrate

[0030] region is impermeable, at least regionally, to the substance to be incorporated. In order to incorporate the substance, the solid body is contacted with a substance, for example in a gas phase, that essentially diffuses only into the exposed substrate regions while the remaining substrate regions remain free of the substance.

[0031]FIG. 6 shows a DMOS transistor device produced according to the method, in which device the substance is a dopant which is incorporated into a p⁺ zone for a freewheeling diode. The doping regions 7 located on both sides of the p⁺ zone 15 are n⁺ source regions which are embedded in a p-doped substrate region 11. This p-doped substrate region 11 is in turn embedded in an n-doped substrate region 12. Also visible in FIG. 6 are gate contacts 13, a passivation layer 4, a source contact 14, and a gate oxide layer 16.

[0032] In the method to produce a solid body 2 with a microstructure 2, the surface of substrate 3 is thus provided with masking layer 6 which is impermeable to the substance to be applied. The substance is then incorporated into substrate regions not covered by masking layer 6. A heat treatment is used to diffuse the substance into a specific substrate region covered by masking layer 6 such that a concentration gradient of the substance is created, proceeding from the edge of the masking layer inward with increasing distance from the edge. Masking layer 6 is subsequently removed to expose the substrate region below it, and a near-surface layer of substrate 3 located in the exposed substrate region is converted by a chemical conversion reaction into a coating 9 which has a layer thickness profile corresponding to the concentration gradient of the substance contained in the near-surface layer. A supplementary treatment is implemented in a subsection of coating 9 in which the thickness of coating 9 has been reduced. 

1. Method for producing a solid body (1) including a microstructure, especially, a semiconductor element, wherein the surface of a substrate (3) is provided with a masking layer (6) which is impermeable to a substance to be applied, and wherein the substance is subsequently incorporated into substrate regions not covered by the masking layer (6), characterized in that a heat treatment is used to diffuse the incorporated substance into a substrate region covered by the masking layer (6) such that a concentration gradient of the substance is created proceeding from the edge of the masking layer (6) inward with increasing distance from the edge, in the substrate region covered by the masking layer (6); that the masking layer (6) is next removed to expose the substrate region below it; that a layer of the substrate (3) near the surface in the exposed substrate region is converted by a chemical conversion reaction into a coating (9) with a layer thickness profile corresponding to the concentration gradient of the substance contained in this near-surface layer; and that a supplementary treatment is implemented in a subsection of the coating (9), the surface of which is smaller than the substrate surface covered by the original masking layer (6), and in which the thickness of the coating (9) is reduced relative to the remaining subsections of the coating (9), in which treatment the substrate region covered by this subsection is exposed, and/or a material is incorporated into this substrate region through the coating (9).
 2. Method according to claim 1, characterized in that in order to remove the masking layer (6), the substrate regions laterally adjacent to the masking layer (6) are covered by an etching mask (8), and the masking layer (6) then contacted with an etching agent; and that the etching mask (8) is preferably created by a chemical reaction in which a near-surface layer of the substrate regions to be covered by the etching mask (8) is converted to an etching mask material.
 3. Method according to claims 1 or 2, characterized in that the etching mask (8) is created during the heat treatment by thermal oxidation of the substrate material in an oxygen-containing atmosphere.
 4. Method according to one of claims 1 through 3, characterized in that the chemical conversion reaction is an oxidation reaction.
 5. Method according to one of claims 1 through 4, characterized in that in the substrate region in which the masking layer (6) has been removed, the near-surface layer of the substrate (3) is converted by a chemical conversion reaction into an electrically insulating coating (9); and that, after the regional removal of the coating (9), a metal coating (10) is electrolytically deposited on the exposed surface of the electrically conductive substrate region.
 6. Method according to one of claims 1 through 5, characterized in that a preferably metallic surface layer is applied to the surface of the solid body (1), and that the adhesive properties of the substrate material and of the coating are matched to the material of the surface layer such that this layer continues to adhere only to the exposed subsection of the substrate region.
 7. Method according to one of claims 1 through 6, characterized in that the near-surface layer of the substrate (3) is converted by a chemical conversion reaction to a coating (9) which is impermeable to a chemical layer to be applied; and that during the supplementary treatment, the substrate region covered by a subsection of this coating (9) is first exposed, and the substance is then incorporated into this substrate region.
 8. Method according to one of claims 1 through 7, characterized in that the solid body (1) is contacted by a medium, especially a gas, and that substrate material present in the exposed substrate region is converted by a chemical reaction with this medium into another material.
 9. Method according to one of claims 1 through 8, characterized in that, after exposing the substrate region, the substrate region is contacted with an etching agent for the substrate material, to which the coating (9) surrounding the substrate region is essentially chemically resistant, in order to insert a depression into the substrate region. 